Supersonic X-plane's unusual inlet performs well in wind tunnel

A series of wind tunnel tests revealed the unusual engine inlet positioning for NASA’s supersonic X-plane meets the performance goals for the Lockheed Martin-designed aircraft, a NASA Glenn Research Center aeronautics engineer says.

The quiet supersonic transport (QueSST) X-plane demonstrator will begin a series of flight tests in 2020 with an inlet placed atop the fuselage and behind the cockpit, a rare configuration for a supersonic aircraft not seen since early 1950s designs, such as the Douglas X-3 Stiletto and Convair F2Y Sea Dart.

The unusual engine placement is driven by the purpose of the QueSST demonstrator, explains Ray Castner, a NASA Glenn engineer, speaking at the Experimental Aircraft Association’s annual event in Oshkosh, Wisconsin on 25 July.

NASA is funding the flight demonstration to evaluate how boom-shaping techniques developed after decades of research affect how humans perceive the acoustic disruption caused by breaking the sound barrier. NASA will present the data to the US Federal Aviation Administration by the mid-2020s, with the hope of persuading the agency to modify or eliminate a half-century-old ban on supersonic flight by civilian aircraft over populated areas.

“Most supersonic aircraft have the engines near the front on the nose or underneath in the clean air flow,” Castner says. “We now have our engine up top and that’s for boom-shielding. That way, the disturbance from the engine goes up, and does not propagate down to the ground and contributes to boom signature.”

NASA’s Glenn Research Center in Cleveland, Ohio, performed 73h of testing of a model of the X-plane in the facililty’s 8ft X 6ft wind tunnel, the first such laboratory tests of such an engine inlet position for a supersonic aircraft of which the agency is aware.

The result satisfied NASA’s engineers that the X-plane’s unique inlet position will work.

“This inlet is actually more efficient than I thought it would be,” Castner says. “It was about 96-98% efficient, so that’s pretty good.”

Although the positioning was different, the nature of the NASA’s QueSST demonstration allowed Lockheed to use a relatively simple inlet design. NASA plans to have the aircraft take-off, make two passes over a city at Mach 1.4, then land. The design includes a diverterless bump to steer boundary layer airflow away from the inlet, but requires no moving pieces required for supersonic aircraft designed to cruise at higher speeds.

“It’s a [sonic] boom demonstrator. It’s not an inlet demonstrator. There is a higher performing inlet that we could have chosen, but a lot of those inlets have moveable parts,” Castner says.

NASA’s concerns about boundary layer flow over the top of the fuselage with the inlet’s placement drove other design decisions, he adds. After Lockheed completed the preliminary design, NASA released an image of the demonstrator with six vortex generators set between the cockpit canopy and the engine inlet. Lockheed placed the vortex generators there to energise the boundary layer flow and prevent the inlet from ingesting that relatively stagnant air, he says.

The Glenn wind tunnel also performed aerodynamic tests of the X-plane model in more than 40 different configurations.

The design is shaped to the reduce the sonic boom disruption to 75db on the ground, compared to 105db for the British Aerospace/Aerospatiale Concorde. That design goal drives the designers to extend the length of the nose almost absurdly. The extra length of the nose reduces the pilot’s forward visibility, so NASA is installing an external vision system (XVS) for the pilot to see straight ahead. Canard surfaces placed just forward of the cockpit to help shape the position of the supersonic shockwave, Castner says.

The aircraft also features small surfaces on the vertical stabilizer that form miniature T-tail: “It’s only job is boom-shaping,” Castner says. “It contributes to the low sonic boom signature.”

NASA plans to release a solicitation to industry for contractors to build the Lockheed X-plane design starting in Fiscal 2018.